It is generally known that protective films (hereinafter including both lower and upper protective films) for an optical recording medium such as a laser disk or the like typically include 20% of silicon dioxide (SiO2) with the remaining part being zinc sulfide (ZnS). This protective film is known to be obtained by carrying out sputtering with the use of a target for forming a protective film for an optical recording medium, i.e., a ZnS—SiO2 based hot-press sinter which includes 20% silicon dioxide (SiO2) with the remaining part being zinc oxide sulfide (ZnS).
However, such a protective film prepared by using a target composed of a ZnS—SiO2 based hot-press sinter causes a problem in that repeatable re-recording performance deteriorates when a laser beam is irradiated to a recording layer to repetitively perform re-recording since S in ZnS of a target composed of a ZnS—SiO2 based hot-press sinter diffuses in the recording layer. Consequently, development of a protective film containing no S has been carried out. Examples of a protective film containing no S, as follows, are known where values are in mol %.
(i) A protective film for an optical recording medium produced by sintering a mixed powder including 10 to 70% of zirconium oxide, and 50% or less (over 0%) of silicon dioxide, and the remainder containing aluminum oxide and inevitable impurities.
(ii) A protective film for an optical recording medium produced by sintering a mixed powder including 10 to 70% of hafnium oxide, 50% or less (over 0%) of silicon dioxide and the remainder containing aluminum oxide and inevitable impurities.
(iii) A protective film for an optical recording medium produced by sintering a mixed powder including 10 to 70% of zirconium oxide, 0.1 to 8.4% of yttrium oxide, and 50% or less (over 0%) of silicon dioxide, and the remainder containing aluminum oxide and inevitable impurities.
(iv) A protective film for an optical recording medium produced by sintering a mixed powder including 10 to 70% of zirconium oxide, and 50% or less (over 0%) of silicon dioxide, and the remainder containing lanthanum oxide and inevitable impurities.
(v) A protective film for an optical recording medium produced by sintering a mixed powder including 10 to 70% of hafnium oxide, and 50% or less (over 0%) of silicon dioxide, and the remainder containing lanthanum oxide and inevitable impurities.
(vi) A protective film for an optical recording medium produced by sintering a mixed powder including 10 to 70% of zirconium oxide, 0.1 to 8.4% of yttrium oxide, and 50% or less (over 0%) of silicon dioxide, and the remainder containing lanthanum oxide and inevitable impurities.
(vii) A protective film for an optical recording medium produced by sintering a mixed powder including 10 to 70% of zirconium oxide, and 50% or less (over 0%) of silicon dioxide, and the remainder containing indium oxide and inevitable impurities.
(viii) A protective film for an optical recording medium produced by sintering a mixed powder including 10 to 70% of hafnium oxide, and 50% or less (over 0%) of silicon dioxide, and the remainder containing indium oxide and inevitable impurities.
(ix) A protective film for an optical recording medium produced by sintering a mixed powder including 10 to 70% of zirconium oxide, 0.1 to 8.4% of yttrium oxide, and 50% or less (over 0%) of silicon dioxide, and the remainder containing indium oxide and inevitable impurities.
In addition, a target for forming protective films for an optical recording medium which have compositions described in the foregoing (i) to (ix) has been developed. This target has the same composition as the protective films for an optical recording medium mentioned in (i) to (ix) above (refer to Japanese Patent Application Laid-Open No. 2005-56545).
The target employs the oxide powders mentioned in the foregoing (i) to (ix) as a raw powder. The raw powders are mixed at a predetermined ratio and combined to prepare a mixed powder. The mixed powders are molded and baked in air or an oxidative atmosphere such as an oxygen atmosphere, thereby producing the targets.